1. Field of the Invention
The present invention generally relates to a VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier) assignment system in an ATM (Asynchronous Transfer Mode) system.
2. Description of the Prior Art
Recently, there has been considerable activity in the development of an ATM system capable of realizing a B-ISDN (Broadband-Integrated Services Digital Network). In the ATM system, all digital information is converted into cells (ATM cells), each having a fixed data length, and cells are transferred via a transmission medium. Each cell consists of a 5-byte header, and a 48-byte information field, and hence consists of 53 octets. The header includes identifiers VPI and VCI. A logical path and a logical channel, via which each cell is transferred, are identified from the VPI and VCI contained in each cell, and switching in nodes, such as exchanges and transmission devices, are controlled so that the identified logical paths and channels can be obtained. The CCITT Recommendations define the number of bits included in the ATM cell header, the above bits being used for representing the VPI and VCI (see CCITT Recommendations I. 361 and 363). Hence, the number of usable paths (VPI) and the number of usable channels (VCI) are respectively limited. Hence, a sufficient number of usable paths and a sufficient number of usable channels cannot be obtained in order to transfer a large amount of data via a route having high traffic.
FIG. 1 is a block diagram for explaining the VPI and VCI in an ATM system, and FIG. 2 is a block diagram of a node provided in the ATM system. FIG. 1 shows a plurality of nodes #1, #2 and #3, which are connected to each other via high-bit-rate digital lines (dedicated lines). VPI#1 indicates a path in a route extending from node #1 to node #3 via node #2 serving as a relay node. Two channels VCI#1 and VCI#2 are provided in the path VPI#1. A path VPI#2 is provided as a path in a route extending from node #1 to node #2. Three channels VCI#3, VCI#4 and VCI#5 are provided in the path VPI#2. It will be noted that the paths VPI#1 and VIP#2 correspond to a single physical transmission line or to several different transmission lines.
Referring to FIG. 2, a terminal 150, such as a telephone set or a data terminal, and a modem (modulator/demodulator) 151 are connected to a node 152. The node 152 comprises a node management unit 153, conversion (cell assembly/disassembly) units 154 and 155, a route switching unit 156, a control bus 157, and a data bus 158. If the terminal 150 generates a call and specifies a remote destination terminal, necessary information is transferred from the terminal 150 to the node management unit 153 via the conversion unit 154 and the control bus 157. The node management unit 153 determines whether or not a VPI and a VCI for making a communication route connected to the destination terminal have already been assigned to the terminal 150. If such a pair of VPI and VCI has been assigned to the terminal 150, the node management unit 153 informs the conversion unit 154 of the result of the above determination. If it is determined that such a pair of VPI and VCI has not yet been assigned to the terminal 150, the node management unit 153 sends an VPI/VCI assignment request to a network management unit (not shown) provided in the ATM system. The conversion unit 154 assembles ATM cells from data transferred from the terminal 150, and the assigned VPI and VCI are written into each of the ATM cells.
The ATM cell header includes a 12-bit (for NNI: Network-Network Interface) or 8-bit (for UNI:User-Network Interface) VPI area, and a 16-bit VCI area, and further includes four bits for payload type and for setting priority and reservation, and eight bits for checking whether or not the ATM cell header has an error. The ATM cells are applied to the route switching unit 156 via the data bus 158. The route switching unit 156 discriminates the VPI and VCI included in each of the ATM cells, and outputs the ATM cells to a digital service unit (DSU) 160 in the corresponding route. Then, the ATM cells are transferred via a high-bit-rate digital line extending from the digital service unit 160.
In the conventional ATM system, the VPI and VCI used in the previous communications continue to be held in the node management unit 153 after the terminal 150 completes the communications. Hence, the terminal 150 can use the VPI and VCI previously assigned and held in the node management unit 153 when the terminal 150 generates a call. Calls from the modem 151 can be processed in the same manner as calls from the terminal 150.
The node management unit 153 is connected, via a digital service unit (DSU) 159, to a node management unit of another node and/or the aforementioned network management unit. The node management unit 153 sends requests to another node management unit and the network management units, and receives a VPI and a VCI assigned by another node.
However, the above-mentioned conventional VPI and VCI assignment system has the following disadvantages. The assigned VPIs and VCIs continue to be held after the communications are completed. Meanwhile, the number of paths and the number of channels are limited to 2.sup.12 (=4096)/2 .sup.8 (256) and 2.sup.16 (=65536), respectively. Hence, it is very difficult to transmit data via a route having high traffic by means of the limited number of paths and channels under the condition that the assigned VPIs and VCIs continue to be held after the communications are completed.
It is necessary to assign paths and channels to bypass routes beforehand rather than executing a process in which idle paths and channels are searched for and assigned in response to the detection of a fault that has occurred in the system. If a fault has occurred, a corresponding bypass route is immediately established. The bypass routes may be used when there is no idle channel. Normally, the paths and channels assigned to the bypass routes cannot be used. Hence, a number of paths smaller than 4096 (NNI cells)/256 (UNI cells) and a number of channels smaller than 65536 can be used in practice.